Recently, it is extremely important to obtain information on an accurate time waveform of an optical signal, in order to put into practical use, for instance, an information communication system using nonlinear optical effects. Now, an intensity distribution of an optical signal is measured using an optical sampling oscilloscope, an auto correlator, and the like, in order to obtain information on accurate time waveforms of the optical signal. As a result, amplitude information of the optical signal is obtained from the measured intensity distribution. However, phase information of the optical signal cannot be obtained from the measured intensity distribution. Specifically, information on a time waveform of the optical signal cannot be obtained by merely measuring the intensity distribution of the optical signal.
Now, various methods for obtaining a phase of an optical signal have been proposed (see Non-Patent Literatures (NPLs) 1 and 2, for example). The methods disclosed in NPLs 1 and 2 are methods for obtaining a phase of an optical signal based on time-resolved spectroscopy. Specifically, according to the methods disclosed in NPLs 1 and 2, a phase of an optical signal is obtained using an ultra high-speed time gate or a reference light source. Then, a time waveform of an optical signal is reconstructed using the obtained phase.